1. Field of the Invention
The present invention relates generally to steam turbines and, more specifically, for providing early warning of potential turbine water induction events and to a system maintaining clearances between rotating and stationary parts thereby enabling steam turbine restarts. The early warning system enables utilities to take timely corrective action to avoid such water induction.
2. Description of the Related Art
Turbine heating blankets have been used to alleviate the potential for damage to turbine components caused by significant temperature differentials, to maintain the turbine unit on warm standby for rapid start-up, and to reduce start-up time and thus improve turbine availability.
A steam turbine restart temperature maintenance system employing heating blankets is described in U.S Pat. No. 4,584,836, issued to McClelland. A steam turbine cover or outer casing is enveloped by a plurality of blankets, each sized and shaped to fit the contour of the turbine and held in place by bolting. The blankets include a plurality of "heater elements" which have a conductive core and a ceramic sheath. The elements are held between a thin corrugated metal layer overlaid on the outer casing of the turbine and two plies of ceramic fiber insulator. When the elements are energized by a power source, heat is transferred to the turbine.
The system described above minimizes cover/base temperature differentials arising from gaseous convection off-line, thus reducing the potential for thermal cylinder distortion including cylinder arching. Cylinder arching can change clearances and can lead to serious rubs between rotating and stationary parts in the blade path, depending on the magnitude of cover/base differential and the turbine configuration. The system has also been found to be well suited to maintaining minimum turbine temperatures during shut down.
The maintenance of proper clearances between stationary and rotating parts is crucial to the prevention of major wrecks or rub damage to seals.
Water inductions pose a major threat to steam turbines since they have the potential for causing substantial thermal stresses and distortions leading to damage to stationary and rotating parts of the turbine. More gradual temperature changes occurring from convection cooling following a turbine trip while the turbine is off-line and on turning gear, can also cause physical damage. Temperature differences between the cover and base of a turbine casing cause thermal deformation and can lead to blade path seal rubs, permanent distortion or more severe damage depending on the magnitude of the temperature differences.
Many turbine water induction incidents develop as a result of a relatively slow accumulation of water (or cool fluid) in interface steam pipes and vessels.
A need exists for an early warning of the abnormal presence of water or cool fluid and abnormal or unacceptable cover/base temperature differences.